Mitochondria are important and dynamic organelles that produce energy for the cell, and mitochondrial defects underlie several human diseases. We have found that loss of the small GTPase ARL2 severely impairs mitochondrial function. Specifically, loss of ARL2 activity causes defects in mitochondrial morphology, motility, and ATP production. We have also found that knockdown of an ARL2 GTPase activating protein (GAP), ELMOD2, also causes defects in morphology, motility, and ATP production. Because both ARL2 and ELMOD2 localize to the mitochondrial matrix, can bind to each other, and produce the same phenotypes upon knockdown, they are likely members of an important mitochondrial signaling pathway. Additionally, loss of ARL2 and ELMOD2 phenocopy the loss of OPA1, an important regulator of cristae, which are critical for mitochondrial ATP production. Therefore, I hypothesize that ARL2 signaling through ELMOD2 in the mitochondrial matrix is required for proper maintenance of mitochondrial morphology, motility, and ATP production, through regulation of cristae morphology. In Aim 1 I will determine if matrix-targeted ARL2 and ELMOD2 can rescue the mitochondrial defects resulting from ARL2 and ELMOD2 knockdown, which will test my model that ARL2 and ELMOD2 act from within the matrix. To test if ELMOD2 acts as an ARL2 effector, I will generate point mutants of ARL2 that cannot bind ELMOD2, which will determine if ARL2 binding to ELMOD2 is required for its mitochondrial functions. I will also use a GAP dead ELMOD2 mutant, ELMOD2[R167K], to determine if ELMOD2 GAP activity for ARL2 is required for mitochondrial function. In Aim 2 I will use electron microscopy to determine the sub-mitochondrial localization of ARL2 and ELMOD2, and test my hypothesis that their knockdown alters cristae morphology. Studying these evolutionarily conserved proteins will inform us about proper mitochondrial function, which can be used for the study of many human diseases.